The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material in which the image is to be reproduced, such as paper or cloth.
Negative-working lithographic printing plates are prepared from negative-working radiation-sensitive compositions that are formed from polymers that crosslink in radiation-exposed areas. A developing solution is used to remove the unexposed areas of the plates to thereby form a negative image.
The most widely used type of negative-working lithographic printing plates comprises a layer of a radiation-sensitive composition applied to an aluminum substrate and commonly includes a subbing layer or interlayer to control the bonding of the radiation-sensitive layer to the substrate. The aluminum substrate is typically provided with an anodized coating formed by anodically oxidizing the aluminum in an aqueous electrolyte solution.
It is well known to prepare negative-working lithographic printing plates using a radiation-sensitive composition that includes a photocrosslinkable polymer containing the photosensitive group, --CH.dbd.CH--CO-- as an integral part of the polymer backbone see for example, U.S. Pat. No. 3,030,208 (Schellenberg et al), U.S. Pat. No. 3,622,320 (Allen), U.S. Pat. No. 3,702,765 (Laakso) and U.S. Pat. No. 3,929,489 (Arcesi et al)!. A very common commercially useful photocrosslinkable polymer in lithographic printing plates is a polyester prepared from diethyl p-phenylenediacrylate and 1,4-bis(.beta.-hydroxyethoxy)-cyclohexane.
Polyesters in addition to Polymer A that are especially useful in the preparation of lithographic printing plates are those which incorporate ionic moieties derived from monomers such as dimethyl-3,3'-(sodioimino)disulfonyl!dibenzoate and dimethyl-5-sodiosulfoisophthalate. Polyesters of this type are well known and are described, for example, in U.S. Pat. No. 3,929,489 (Arcesi). A preferred polyester of this type, known in the art as "Polymer B", is poly1,4-cyclohexylene-bis(oxyethylene)-p-phenylenediacrylate!-co-3,3'(so dioimino)disulfonyl!dibenzoate. Another preferred polyester of this type is known in the art as "Polymer C" and is defined as poly1,4-cyclohexylene-bis(oxyethylene)-p-phenylenediacrylate!-co-3,3'-(s odioimino)disulfonyl!dibenzoate-co-3-hydroxyisophthalate.
Various other photocrosslinkable polymers are known for use in lithographic printing plates including the polyimide precursors described in U.S. Pat. No. 4,416,973 (Goff). Such compositions are known for durability, chemical resistance, adhesion to metals and high mechanical strength when photocrosslinked. Well-known sensitizers including coumarins and halogenated triazines, as described for example in U.S. Pat. No. 4,505,793 (Tamoto et al), can also be used in photocrosslinkable compositions.
There is a need to provide a highly durable negative-working lithographic printing plate that is simple to make and use. Thus, it would be desirable to have plates having a single layer which is a photocrosslinkable layer that has very high sensitivity.
Unfortunately, photopolymerizable compositions using acrylate monomers and photocrosslinkable polymers are oxygen sensitive. Consequently they tend to exhibit low photospeed and low contrast. To overcome these undesirable characteristics complicated oxygen scavenging schemes have been proposed. Very commonly a relatively thick polyvinyl alcohol overcoat is employed as an effective oxygen barrier layer. This additional step however, adds cost and complexity to the manufacturing and processing of the plate coating.